1. Field of the Invention
The present invention relates to a discharge tube having a positive glow discharge characteristic in which the tube voltage increases as the discharge current increases and a negative arc discharge characteristic in which the tube voltage decreases as the discharge current increases due to an increase in emission of thermo-electrons.
2. Description of the Related Art
It is known that cold-cathode tubes, hot-cathode tubes and semi-hot cathode tubes are used in discharge-tube applications.
Cold-cathode tubes have positive discharge characteristics and have the advantages of long life, low power consumption, low heat dissipation, and the ability to be easily lit and quenched. The disadvantage of cold-cathode tubes is their low intensity. Hot-cathode tubes, which are known as fluorescent lamps, have negative thermionic discharge characteristics and the advantage of high intensity. Hot-cathode tubes, however, have the disadvantages of short life, high power consumption, high heat dissipation, and cannot be lit and quenched in themselves. Semi-hot-cathode tubes are arranged such that no filament electrode is energized by external circuits and have the disadvantage that a relatively long time is required to reach the desired intensity after energization. Also, semi-hot-cathode tubes are impractical in terms of life and intensity.
In Japanese Patent Application No. 63-172761, the present inventor proposes a discharge tube of the type having a discharge characteristic in which the negative discharge characteristic of a cold-cathode tube is combined with the positive discharge characteristic of a hot-cathode tube. More specifically, the proposed discharge tube comprises a pair of electrode assemblies which are disposed opposite to each other in a discharge space. Each of the electrode assemblies includes a cup-shaped electrode for glow discharge and, a filament electrode for arc discharge which is disposed in the cup-shaped electrode. This arrangement is intended to achieve a long life with glow discharges and very high brightness with arc discharges. However, it is necessary that such discharge tube be provided with an automatic control circuit for controlling discharge current in order to stably maintain the state of discharge. In this proposed type of discharge tube, several hours after energization the temperature of the cup-shaped electrode for glow discharge increases due to emission of thermoelectrons and a transition from glow discharge to arc discharge occurs. As a result, a phenomenon such as snaking or flickering takes place and the discharge becomes unstable. For this reason, the above electronic control circuit is used to control discharge current. However, the cost of the discharge tube becomes expensive due to the necessity of such control circuit and, even with this control circuit, it is still difficult to perfectly eliminate flickers of high frequency.